I'm guessing you would do this in advance - "pay attention to tech news next week - our botnet will unleash hell"
Step 1: Put message on blockchain beforehand with exact date/time and characteristics of DDoS
Step 2: Execute DDoS
Step 3: Prove to others you are responsible by using private key
There’s (as ever) no need for a blockchain, people do this with twitter and sha256 all the time. Hash the message, post the hash, wait for prediction to happen, post the full message.
Any examples of major predictions verified in this way?